The present invention relates generally to a system for measuring the set pressure of a valve and, more particularly, to a lightweight and portable set pressure verification device for use in conjunction with spring loaded safety relief valves. The device has particular application for use in nuclear power plant containment facilities.
Pressure lines are often designed with safety valves located at various positions along their length. Conventional valves, such as, for example, the Type HB valves manufactured by Crosby Valve and Gage Company, Wrentham, Mass., include a closure element which is nominally biased against the port to seal the port. Typically, this bias is provided by a springloaded assembly coupled to the closure element. The safety valve is designed so that when the pressure in the line is below a predetermined threshold (the "set pressure"), the valve port is sealed. When, however, the line pressure exceeds the set pressure and is sufficient to overcome the spring bias, the valve lifts from its seat in the valve port and the medium within the pressure line is able to freely escape. In the prior art, safety valves are designed for specifically desired set pressures for various applications.
A particular problem experienced in the use of such safety valves was the need to ensure that the set pressure of the valve, as installed, did not change with time, or, if it did change, that the change could be measured. Due to the fact that such set pressures do periodically change, most users will periodically test the set points of their safety valves. One approach for measuring a valve's set point was to remove the valve from the pressure line assembly, fit it on a test jig, test the set point, and then either re-install or replace the valve, as required. This approach, of course, necessitates a shutdown of the pressure line during set point testing.
An alternative test procedure fits the valve with an air-operated lifting motor which is coupled to the closure element and a pressure gauge. In operation, a test operator applies air pressure to the air motor by way of an air pressure regulator or a manual control valve. The resultant force from the motor is applied to the closure element to overcome the springloaded bias and thereupon lift the closure element. When the lifting force from the motor together with the force from the pressure within the line is sufficient to overcome the springloaded bias force, the closure element lifts off its seat with an accompanying audible leak "pop". When such a "pop" is detected by the operator, the air pressure applied to the air motor is measured, and a differential force value may be derived from a predetermined relationship between those parameters for the valve. This differential pressure may be added to the pressure within the line to yield the set pressure characteristic for the valve.
Recordation of this audible "pop" is not an acceptable test procedure in many applications, particularly in a relatively noisy environment where the test operator may not be able to detect the "audible pop". Furthermore, in some applications, such as in boiling water reactor plants, there are hazardous radiation environments in which the set point test operator may not safely enter the immediate region of the valve-to-be-tested. In the prior art, there have been attempts to utilize the above described "audible pop" technique with an acoustical transducer so that remote set point testing may be performed for valves in operational systems. However, these attempts have been unsuccessful, principally due to background noise.
More recent attempts have focused on the use of strip chart recorders and computer assisted recording devices to measure set pressure. Heretofore, however, there has been a tradeoff in such valve testing devices between portability and accuracy. The most reliable of the testing devices have been of the type which are either permanently or semi-permanently mounted on the valve. See, for example, commonly-assigned U.S. Pat. No. 4,349,885, which issued on September 14, 1982 in the name of Leonard J. Thompson for Set Pressure Measuring System, which relies on the use of an external controller to record and process a number of different signals to determine set pressure. This system has proven extremely accurate and is commercially successful. Its construction, however, requires that it either be permanently or semi-permanently mounted on a valve.
A more portable type of testing device is described in U.S. Pat. No. 4,428,223 which issued on Jan. 31, 1984 in the name of Ottavio Trevisan for Apparatus for Periodically Testing the Operation of Safety Valves. Such a system, however, lacks the degree of accuracy of the testing system of the earlier Thompson patent.
Against the foregoing background, it is a primary object of the present invention to provide a relatively portable set pressure measurement device.
It is another object of the present invention to provide such a device which is able to be readily mounted and dismounted on a valve to increase the number of valves which can be practically tested in a fixed period of time.
It is still another object of the present invention to provide such a device which imposes a lower appurtenance load on the valve which it is adapted to test.
It is yet still another object of the present invention to provide such a device which is lower in cost and has fewer movable parts than the test devices heretofore used.
It is still yet another object of the present invention to provide such a device the accuracy of which is equal to the permanently or semi-permanently mounted devices heretofore used.